1. Field of the Invention
This invention relates to a technique for printing an image on a print medium by ejecting ink thereon.
2. Description of the Related Art
In recent years, color printers of a type wherein inks of several colors are ejected from a print head have come to enjoy widespread use as computer output devices, and are widely used for multi-color, multi-level printing of computer-processed images. Such tone representation is performed by a method in which one of several types of dots of different sizes is selectively formed in a one-pixel area of a print medium.
With such a tone representation method, each tone value is represented by recording dots of each size at each appropriate recording rate. This dot recording rate is preset to an optimal value so as to minimize graininess (deviation of image) and banding (band-like image degradation).
However, where error is present in ink ejection quantity, error occurs in dots of each size. Where error occurs in dots of each size, there occurs the problem of dot recording rate, which is preset on the presumption of each dot size, no longer being the optimal value.
Accordingly, an object of the present invention is to reduce graininess and banding due to ink ejection quantity error.
In order to attain the above and the other objects of the present invention, there is provided a printing apparatus capable of forming one of N types of dots having different sizes at one pixel area by a selective ejection of any of the N types of ink drops onto a print medium. The N types of ink drops are different in an ink amount. N is an integer of at least 2. The apparatus comprises an error information receiver, a dot data generator, and a dot recorder. The error information receiver is configured to receive error information indicative of an ink amount error of at least one specific type of ink drop among the N types of ink drops. The dot data generator is configured to process input image data for generating dot data representing a state of dot formation at each pixel in a printed image. The dot recorder is configured to eject the N types of ink drops onto the print medium in response to the dot data. The dot data generator is configured to generate the dot data whose specific dot recording rate is adjusted in response to the error information. The specific dot recording rate is a rate of dot recorded with the specific type of ink drops.
According to the printing device of the present invention, the recording rate of at least one type of dot among a plurality of types of dots is adjusted in response to error information, whereby the occurrence of graininess and banding due to ink ejection quantity error can be reduced.
In a preferred embodiment of the invention, the specific type of ink drop is an ink drop to record a relatively small dot among the N types of ink dots, and the dot data generator is configured to generate the dot data whose upper limit of the specific dot recording rate is modified.
The maximum value for recording rate of relatively small dots among several types of dots has a relatively large effect on occurrence of banding, and therefore by modifying the upper limit value for recording rate of such dots, graininess and banding can be effectively held in check.
In a preferred embodiment of the invention, the dot data generator is configured to raise the upper limit of the specific dot recording rate when the error information shows increase of the ink amount by the ink amount error.
If ink drops of relatively small dots become large, there is a tendency for banding to become difficult to occur even if the recording rate of those dots is increased, and therefore the upper limit of recording rate of such dots can be increased and the recording rate of relatively large dots that are readily visible can be decreased. As a result, occurrence of banding can be held in check while minimizing graininess, and print quality can be improved.
In a preferred embodiment of the invention, the dot data generator is configured to lower the upper limit of the specific dot recording rate when the error information indicates decrease of the ink amount by the ink amount error.
If ink drops of relatively small dots become small, there is a tendency for banding to easily occur by means of recording of such dots. However, in such instances as well, by adopting the arrangement described above, the upper limit for recording rate of such dots can be reduced and the occurrence of banding held in check.
In another embodiment, the specific type of ink drop has a lowest ink amount among the N types of ink drops.
In a preferred embodiment of the invention, the dot data generator comprises a dot recording rate table storage and a dot recording rate table selector. The dot recording rate table storage is configured to storage a plurality of dot recording rate tables for determination of the N types of ink dots. The dot recording rate table selector is configured to select one of the plurality of dot recording rate tables in response to the error information. The plurality of dot recording rate tables includes a specific table to be selected in response to the error information.
By so doing, dot recording rate can be adjusted simply by selecting a dot recording rate table depending on error information, and therefore the burden of processing associated with implementation of the present invention is minimal. Further, the present invention can be implemented without modifying the method of the halftone process or other decrease process, so the present invention can be implemented easily.
In a preferred embodiment of the invention, the dot data generator is configured to generate the dot data compensated for the ink amount error of the specific type of ink drop in response to the error information.
In a preferred embodiment of the invention, the specific dot recording rate table is further adjusted to compensate for the ink amount error of the specific type of ink drop in response to the error information.
By so doing, graininess and the like can be improved while at the same time compensating error in ink quantity, making it possible to reproduce accurate color.
In a preferred embodiment of the invention, the dot data generator is configured to generate the dot data whose specific dot recording rate is adjusted in response to the error information and a type of print medium.
By so doing, even in instances where the preferred dot recording rate differs for each individual print medium, it is possible to come close to the preferred dot recording rate for each print medium.
The present invention can be realized in various forms such as a method and apparatus for printing, a method and apparatus for producing print data for a printing unit, and a computer program product implementing the above scheme.